Extensive information is available from official statistics and descriptive studies on the association between different socio-demographic background factors and sickness absence. This information addresses age, gender, place of residence, and socio-economic status. However, few studies have thoroughly analysed these background factors, and rigorous scientific evidence on the causal relationship between these factors and sick leave is lacking. Regarding the family, we found no scientific evidence that marital status or children living at home were associated with sickness absence. However, we found limited scientific evidence for an effect of divorce. Regarding work-related factors, we found limited scientific evidence for an effect of physically stressful work, and moderate scientific evidence for low psychological control over the work situation. We found limited scientific evidence for a correlation in time between unemployment and sickness absence, but insufficient scientific evidence for the causes of the association. There was moderate scientific evidence that the amount of sickness absence is influenced by the design of the social insurance system, but insufficient evidence on the magnitude of change required to influence the level of sickness absence. Essentially the same results apply to disability pension, although the number of studies is small. However, we found moderate scientific evidence for the effects of socio-economic status, which could be explained partly by childhood experiences.